So, this means all java based websites will be unreachable by Opera?

Nope, but their java applet will no longer work in Opera, Chrome, Vivaldi and all Chromium based browsers with current versions of Chromium.

Also, Java plugin is being discontinued By Oracle. It means that at a certain point until the end of this year, Java will probably stop working on all browsers.

IMHO, sooner or later plugins will be part of the past.

So, this means all java based websites will be unreachable by Opera?
Don't you think there is a risk Opera will loose a lot of its userbase?
There're still a lot of websites with Java...

And where they'll move to?

Java doesn't work in Chrome already (and include all Chromium-based browsers). Microsoft Edge, same. Firefox will drop NPAPI too.

Websites will have to be updated to stop using Java (browsers are warning them of NPAPI's discontinuation for almost 3 years or more). Legacy Java web apps will have to be accessed with Internet Explorer or outdated browser versions, and user will use other browser of choice to browse the rest of the web.

Use the extension for the translation of web pages.

There is no meaning in point to people a discussion in a language different of the one being used in the forum in which the question was made.

Sama Oracle announced the discontinuation of the Java browser plug-in your own Java 9 comes with these plug-ins , watch the news here "Moving to a Plugin-Free Web"